Future U.S. elections may very well face more Russian attempts to interfere with the outcome, the FBI and the National Security Agency warned on Monday.

“They’ll be back,” said FBI director James Comey. “They’ll be back in 2020. They may be back in 2018.”

Comey made the comment during a congressional hearing on Russia’s suspected efforts to meddle with last year’s presidential election. Allegedly, cyberspies from the country hacked several high-profile Democratic groups and people, in an effort to tilt the outcome in President Donald Trump’s favor.

Although Russia has denied any involvement, the FBI expects the country to strike again.

“One of the lessons they [Russia] may draw from this is they were successful,” Comey said. “Because they introduced chaos and division and discord.”

Both the FBI and NSA are working with their counterparts in Europe to help stop similar election-tampering efforts allegedly from Russia, Rogers said.

France and Germany are two countries about to hold their own elections, he noted.

“Disinformation, fake news, attempts to release information to embarrass individuals. You’re seeing that play out to some extent in European elections right now,” Rogers said.

Back in December, a German intelligence agency accused Russia of ramping up online propaganda and hacking efforts to influence politics in the country.

The tactics have included spear phishing email attempts against German political parties that appear to resemble those carried out by Russian cyberspies during the U.S. presidential campaign, the agency said.

In February, Norway also said spear phishing attacks had targeted email accounts belonging to political officials in the country -- those, too, similar to tactics used by a suspected Russian hacking group.

In response, some are urging NATO to fight back. “Russia is clearly testing NATO and the West,” U.K. Defense Secretary Michael Fallon said last month.

In the U.S., lawmakers have launched investigations into Russia’s suspected role in influencing last year’s election.

But cybersecurity experts say the country needs to seriously bolster its cyber defenses.

The U.S. needs to stop being so “reactive” to every cyberattack and work on ways to punish offenders and protect private companies from intrusions, said Mark Kuhr, CTO at security firm Synack.

“This secret cyberwar never used to be in the news, but now its leaked out every other week,” he said. “We’ve got to start deterring these attacks.”